Actually swing states are states where the presidential race is too close to call.

The presidential election is decided by the Electoral College. Each state can determine their own method for deciding who casts the Electoral College votes. In all but two states, the people casting the Electoral College votes are determined by which candidate wins the most votes in that state. If a candidate wins Florida, all 27 of Florida's Electoral College votes are cast by supporters of the winning candidate. Quite a few states even require by law that the electors vote for the candidate that won the state's election.

In a close election, the winner can be decided by a small 'swing' in percentages in just a few states where the candidates are too close to call. If one candidate is going to get 70% of the votes in a given state, neither candidate spends much time campaigning in that state, since the outcome is unlikely to change. If the percentages are 51%-49% or so, just a little extra effort campaigning can change the opinion of enough people to change the balance to 49%-51%. In a state like Florida, with 27 Electoral Votes, that can make a big difference. In a really close election, even a smaller state with 9 or 10 Electoral Votes can make a difference.